In general, display devices are broadly classified into light-emitting display devices, such as a cathode ray tube (CRT), a plasma display panel (PDP), a light-emitting diode (LED), and an organic light-emitting diode (OLED), and non-emissive display devices such as a liquid crystal display (LCD). The former has a disadvantage in that a displayed image is vaguely shown in a light space although it has a fast response speed, and the latter has a disadvantage in that the response speed thereof is slow although a displayed image is clearly shown in the light space. Among these display devices, an electrochromic device (ECD) is a type of non-emissive display device, like the LCD. The electrochromic device is a display device which adjusts the color of electrochromic material by controlling a chemical reaction through the application of an electric signal.
Meanwhile, FIG. 1 is a cross-sectional view schematically illustrating the basic structure of an electrochromic device. Referring to FIG. 1, the electrochromic device 1 includes a first electrode 11 formed on a first glass substrate 10, and a second electrode 16 formed on a second glass substrate 17, in which the first and second electrodes 11 and 16 are spaced while facing each other. In addition, the electrochromic device 1 includes an electrochromic layer 12, an electrolyte layer 13, and an ion storage layer 15. The electrochromic layer 12 is formed on the upper surface of the first electrode 11 and includes electrochromic material, the color of which changes depending on an applied electric signal. The electrolyte layer 13 is formed on the top of the electrochromic layer 12 and includes ions concerned in an electrochromic reaction. The ion storage layer 15 is formed on the second electrode 16 and gathers ions having a polarity opposite to that of the ions concerned in the electrochromic reaction.
Generally, at least one of the first and second electrodes 11 and 16 employs a transparent electrode, for example, an indium-tin oxide (ITO) electrode. The ion storage layer 15 may be omitted.
When forward voltage is applied to the electrochromic device 1 so as to let an electric current flow from the electrochromic layer to the ion storage layer, the electrochromic layer 12 is colored, and when reverse voltage is applied to the electrochromic device 1 so as to let an electric current flow from the ion storage layer to the electrochromic layer, the electrochromic layer 12 is bleached. Meanwhile, according to the material of the electrochromic layer, the electrochromic layer may be colored or bleached by the flow of electric current in the opposite directions.
FIG. 2a is a view illustrating the layout of an electrochromic layer for displaying numbers in a 7-segment pattern, and FIG. 2b is a view illustrating the layout of a conventional electrode for applying an electric signal to the electrochromic layer of FIG. 2a. FIGS. 2a and 2b show an example of a direct-driving type electrochromic display device in which separate electrodes are connected to each segment or each pixel, respectively.
Meanwhile, an active-matrix-driving type electrochromic display device is constructed, for example, such that a number of electrochromic pixels are mounted on a panel equipped with a switching element for each pixel. Since the on/off operation of each pixel in such an electrochromic display device is independently controlled, the electrochromic display device has advantages in that it can realize a relatively higher image quality and a relatively faster response speed, and has excellent resolution and moving picture realization capability. In this case, it is well known that a thin film transistor (TFT) is used as the switching element.
FIG. 3a is a view illustrating the structure of an electrode of a thin film transistor in a typical active-matrix-driving type electrochromic device. Herein, an electrode 21 for applying a signal directly to a pixel corresponds to the first electrode 11 of FIG. 1.
FIG. 3b is a cross-sectional view of the thin film transistor shown in FIG. 3a. A direct-driving type device and an active-matrix-driving type device have structures similar to each other, except for a TFT existing beneath the first electrode.
FIG. 3c is a view illustrating the simplest structure obtained when the electrochromic device of FIG. 1 is formed on the TFT of FIG. 3b. 
Meanwhile, observer's eyes to the electrochromic device are oriented toward a second substrate 27. Therefore, the observer's eyes need to pass through an ion storage layer 25 and a second electrode 26 in order to see a displayed color. Then, the contrast ratio is degraded as much.
Particularly, when the ion storage layer 25 is a colored layer, a color reproduction rate as well as a contrast ratio is degraded.